1. Field of the Invention
The invention relates to the control of a vehicle mounted with a transmission that continuously changes the transmission speed of a motive power of an internal combustion engine and transmits the motive power to driving wheels, and more particularly, to an art of making an improvement in fuel consumption while suppressing the generation of vibrations and noise in an internal combustion engine.
2. Description of the Related Art
As a vehicle mounted with a transmission that continuously changes the transmission speed of a motive power of an internal combustion engine and transmits the motive power to driving wheels, there is known, for example, a hybrid vehicle or a vehicle mounted with a continuously variable transmission. In a vehicle mounted with such a transmission, a target power of an internal combustion engine is decided from an accelerator opening degree and a speed of the vehicle, and the internal combustion engine and the transmission are controlled such that the target power emerges on an optimal fuel consumption operation curve.
As a vehicle mounted with such a transmission, for example, Japanese Patent Application Publication No. 2007-22296 (JP-A-2007-22296) discloses a hybrid vehicle that does not give a driver a sense of incongruity resulting from an abnormal noise such as a muffled sound of an internal combustion engine or the like while making a further improvement in energy efficiency. This hybrid vehicle is equipped with the internal combustion engine, electric power/motive power input/output means connected to an output shaft of the internal combustion engine and an axle side to make it possible to output at least part of a motive power from the internal combustion engine to the axle side while having an electric power and the motive power input thereto and output therefrom, an electric motor that outputs a motive power for running, storage means capable of exchanging an electric power with the electric power/motive power input/output means and the electric motor, required driving force setting means for setting a required driving force required for running, noise level detection means for detecting a level of noise based on running, operational restriction setting means for setting an operational restriction imposed on the operation of the internal combustion engine on the basis of the detected level of noise, and control means for controlling the internal combustion engine, the electric power/motive power input/output means, and the electric motor such that the internal combustion engine is operated on the basis of the set operational restriction and that the hybrid vehicle runs by a driving force based on the set required driving force.
According to the hybrid vehicle disclosed in the aforementioned publication, the driver can be restrained from feeling a sense of incongruity due to an abnormal noise such as a muffled sound or the like. As described above, the operational restriction is set in accordance with the level of noise. Therefore, the degree of freedom of the operational restriction set in accordance with the level of noise can be increased. Accordingly, the operational restriction can also be set such that the efficiency of the internal combustion engine for an output is enhanced without giving the driver a sense of incongruity resulting from an abnormal noise such as a muffled sound or the like.
However, in the hybrid vehicle disclosed in the aforementioned publication, an operation point set for an output corresponds to a rotational speed and a torque that increase and decrease respectively as the speed range of the vehicle shifts to a low vehicle speed range. Therefore, the internal combustion engine operates outside a low fuel consumption range, and there is hence a deterioration in fuel consumption. The deterioration in fuel consumption results from a deterioration in the thermal efficiency of the engine, and moreover, the temperature of exhaust gas rises. As a result, the temperature in an engine room may rise to cause a deterioration in overheat tolerance.
Further, in the case where the engine is operated in a low vehicle speed range with good fuel consumption, vibrations with a high frequency and a large amplitude may occur, or the frequency of the occurrence of knock increases in a high load range. The levels of noise and vibrations generated in the internal combustion engine may thereby increase to make the driver feel uncomfortable.